purge-docker-cluster.yml: Default lvm_volumes
purge-docker-cluster.yml: Default lvm_volumes

We were failing when that variable is unset; purge-cluster.yml contains
this workaround.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible